This invention relates to a method and apparatus for obtaining measurements of normal pressure and shear pressure developed by an object; using a single sensor. In a preferred embodiment, an array of sensors are used to measure three dimensional pressures in one operation. The invention can be used to acquire a high resolution array of three dimensional tire footprint pressures.
Some tactile sensing systems have attempted to directly exploit the piezoelectric effect of PVDF film to measure force. The dynamic nature of that effect, however, does not permit static force measurements. Bonneville Scientific uses the PVDF film as a transmitter and receiver of ultrasonic pulses, allowing continuous measurement of a steady force.
Tactile sensors, as illustrated by Bonneville Scientific, use a 2-D array of ultrasound transmitters and receivers to measure the thickness of an overlying elastomer pad that is compressed when it is contacted by an object. The amount of the compression depends on the magnitude of the force applied by the object and the stiffness of the elastomer. The thickness measurement principle is analogous to ultrasonic pulse-echo ranging used in medical imaging and to SONAR for marine navigation and ranging.
Each element of the sensor array transmits an ultrasonic pulse that travels through the elastomer pad, is reflected off the top surface of the pad, and returns to the element. The time interval for this travel, or time-of-flight (TOF), is measured for each element. When a contacted object applies a force normal to the elastomer pad, the pad is compressed and the time-of-flight is reduced. The difference in time-of-flight is proportional to the compression of the pad, which, in turn, is proportional to the applied force. This relationship can be stated as: EQU d.sub.1- d.sub.2 1/2c(t.sub.1 -t.sub.2) (1) EQU F=k(d.sub.1 -d.sub.2)=1/2kc(t.sub.1 -t2) (2)
where:
d=thickness of the pad over the element PA0 t=time-of-flight PA0 F=compressing force PA0 c=speed of sound in the elastomer PA0 k=elastomer stiffness. PA0 d1=the original thickness of the pad over the element PA0 d2=the thickness of the compressed pad PA0 t1=time-of-flight of ultrasound through the noncompressed rubber PA0 t2=time of flight of ultrasound through compressed rubber PA0 F=compressing force PA0 c=speed of sound in the elastomer, and PA0 k=elastomer stiffness.
A laminated sensor produced by Bonneville Scientific is rugged, has no moving parts, and is able to withstand loads of up to 2500 psi. It is highly accurate and gives repeatable results because time-of-flight rather than amplitude is measured, and drift in element or amplifier sensitivity is unimportant. Force measurements can be made every 4-6 .mu.s.
The sensitivity or force range of the sensor can be altered by changing pads. A wide variety of elastomers can be used in the pads, and pads can be replaced inexpensively when they become worn, damaged, or contaminated.
The Bonneville device consists of the sensor array, an electronics module, a computer interface board, and a remote microcomputer. The sensor array, which lies on a 0.04 inch thick substrate, is a PVDF film with 256 elements measuring 0.060 inch by 0.060 inch each. The elements are arranged in 16 columns by 16 rows, with 0.010 inch separation between elements in each direction.
The elastomer pad in the Bonneville device consists of a urethane sheet that determines the sensor's force characteristics and an overlaying thin foam layer that provides a controlled interface for reflecting the ultrasonic pulse. The urethane is available in a range of stiffness from very soft to very firm; the choice is dictated by the application.
The system is operated through a computer keyboard or a mouse. Before operation, a "zeroing" procedure is performed so that the change in the thickness of the elastomer sheet can be calculated when the sensor contacts an object. The sensor is calibrated for force by entering the appropriate value for the stiffness of the elastomer sheet through the keyboard. To show force value, the numerical value of each tactile element (taxel) in the selected tactile image can be displayed.
Various methods and technologies have been used in the past to acquire normal tire footprint pressure distributions. They range from single sensors using microgaging wheatstone bridge technology to pressure sensitive resistors and films such as Fugi film.
Some piezoelectric materials can act as transducers. Piezoelectric transducers convert electrical signals to physical motion. Sensor material is usually a ceramic based on titanates of barium or lead zirconite. The rigidity of ceramics makes them suited for converting electrical energy to mechanical motion in applications such as speakers or signaling alarms but are brittle and cannot be made with large surface areas or in complex shapes. When acting as an ultrasonic receiver/transmitter, the PVDF film acts as a transducer.
Conversly, some piezoelectric materials are able to convert physical motion into an electrical voltage. As used in the present invention, the PVDF film converts pressure contact of the sensor into a measureable voltage.